The present invention relates to nanotube shaping. In particular, the invention relates to a nanotube that exhibits a controllably shaped or xe2x80x9cmade-to-orderxe2x80x9d geometric contour and to a method for shaping a nanotube material having a layered structure.
Microfabrication represents a high-priority research area within a wide range of fields such as electronics, mechanical devices, chemical processes and biological systems. As a result, a number of techniques have been developed to shape the contour of microscopic, three-dimensional solid articles. Such techniques include both additive and subtractive processes, often involving lithographic techniques. However, lithographic techniques are usually insufficient to produce nanometer-sized features. In addition, the accuracy and precision formation of nanometer-sized features is strongly dependent on both the crystallographic structure of the material to be shape as well as the technique employed. Thus, it is evident that only certain techniques may be employed to shape articles of a certain material when high accuracy and close tolerances are required.
Carbon nanotubes are attractive candidates for a host of applications due to their unique mechanical and electrical properties. For example, carbon nanotubes have found use in catalytic reactions, see Freemantle (1996), xe2x80x9cFilled Carbon Nanotubes Could Lead to Improved Catalysts and Biosensors,xe2x80x9d Chemical and Engineering News 74:62-66, electrodes, see Britto et al. (1996), xe2x80x9cCarbon Nanotube Electrode For Oxidation of Dopamine,xe2x80x9d Bioelectrochemistry and Bioenergetics 41:121-125, nanoscale electronics, see Collins et al. (1997), xe2x80x9cNanotube Nanodevice,xe2x80x9d Science 278:100-103, nanoscale mechanical systems, see Iijima (1998), Proc. IEEE Eleventh Annual International Workshop on Micro Elector Mechanical Systems (IEEE, Heidelberg, Germany) 520-525, and scanned probe microscope and electron field emission tips, see Dai et al. (1996), xe2x80x9cNanotubes as Nanoprobes in Scanning Probe Microscopy,xe2x80x9d Nature 384:147-150 and deHeer et al. (1995), xe2x80x9cA Carbon Nanotube Field-Emission Electron Source,xe2x80x9d Science 270:1179-1180. In addition, a number of patents describe various processes that alter the material characteristics of carbon nanotubes, such as functionalization of nanotube surfaces, see, e.g., U.S. Pat. No. 6,203,814 to Fisher et al., or encapsulation of materials in carbon nanotubes, see, e.g., U.S. Pat. No. 5,916,642 to Chang.
Carbon nanotubes have found use as probes for sensing and manipulating microscopic environments and structures. For example, U.S. Pat. No. 6,159,742 to Lieber et al. describes a carbon-based tip that may be used to reveal chemical characteristics of a sample for scanning probe microscopy. The tip is described as having a structure of the formula: Xxe2x80x94(Lxe2x80x94M)n in which n is 1 to 100, X is a carbon-based nanotube having a first end and a second end, L is a linking group bonded at the first end of the carbon-based nanotube, and M is a molecular probe bonded to the linking group. The second end of the carbon-based nanotube X is adapted for attachment to a cantilever configured for microscopy. The linking group L may be a functional moiety such as an amino, amido, carbonyl, carboxyl, alkyl, aryl, ether, or ester group. While this patent describes the attachment of a nanotube to another solid article, the nanotube itself is not controllably shaped.
Similarly, U.S. Pat. No. 6,239,547 to Uemura et al. describes a method for forming an electron-emitting source in which the carbon nanotubes are fixed to a substrate. This method involves preparing a paste by dispersing, in a conductive viscous solution, a plurality of needle-like structures each made of an aggregate of carbon nanotubes. A pattern of this paste is formed on the substrate. Non-needle-like portions are removed to a predetermined degree from the surface of the pattern through laser irradiation or plasma processing, to at least partially expose the needle-like structures. As a result, an electron-emitting source in which the carbon nanotubes are fixed to the substrate is formed. Thus, while this subtractive method allows a nanotube to be attached to a substrate so that the needle-like structure of the nanotube is exposed, the material removal process does not controllably shape the nanotube.
Thus, for use as probes or other applications, it would be desirable to control or shape nanotube geometry. Although recent progress has been made in the growth of nanotubes at pre-selected sites, see Kong et al. (1998), xe2x80x9cSynthesis of Individual Single-Walled Carbon Nanotubes on Patterned Silicon Wafers,xe2x80x9d Nature 395:878-881, and the modification of nanotube ends through chemical etching, see Tsang et al. (1994), xe2x80x9cA Simple Chemical Method of Opening and Filling Carbon Nanotubes,xe2x80x9d Nature 372:159-162, fine control over nanotube shaping has not been possible. When nanotubes are grown, they exhibit a substantially constant cross-sectional dimension along their longitudinal axis. Therefore, while it is possible to grow a plurality of nanotubes of differing diameters, each nanotube exhibits only one diameter. Similarly, known chemical etching techniques do not provide adequate control over the shaping of the contour of a nanotube. While chemical etching removes material from nanotube, targeted material removal from a specific location on the nanotube is currently beyond the capability of those skilled in the art. Often, nanotubes comprising a layered material undergo uncontrolled exfoliation when exposed to an etchant.
There is therefore a need for a method of shaping nanotubes to exhibit a desired or made-to-order contour, particularly nanotubes having a layered structure.
Accordingly, it is an object of the present invention to overcome the above-mentioned disadvantages of the prior art by providing a method for the controllable shaping of nanotubes to exhibit a desired contour.
It is another object of the invention to provide nanotubes that exhibit a controllably shaped contour.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned through routine experimentation upon practice of the invention.
In one embodiment, the invention relates to a method for shaping a nanotube through the use of a shaping electrode to remove material from a portion of the nanotube so that the nanotube is controllably shaped to exhibit a desired contour. The material removal is carried out while the nanotube and the shaping electrode are under a potential difference. Typically, the potential difference is no more than about 10 volts. However, the potential difference is preferably no more than about 5 volts and is optimally about 0.5 to about 3.0 volts. In addition, it is preferred that the potential of the nanotube is at or near ground.
Depending on the potential difference, material removal may take place when the shaping electrode contacts the nanotube or when the shaping electrode is sufficiently close to the nanotube. Thus, material removal may not require contact between the shaping electrode and the nanotube. In some instances, material removal can be carried out by first placing a shaping electrode in contact with the nanotube when the shaping electrode and the nanotube do not exhibit a sufficient potential difference for material removal from the nanotube, and then controllably increasing the potential difference between the electrode and the nanotube to remove material from a portion of the nanotube, thereby shaping the nanotube to exhibit a desired contour. In other instances, the shaping electrode is placed in a noncontacting yet shaping spatial relationship with the nanotube when the shaping electrode and the nanotube do not exhibit a sufficient potential difference for material removal and then controllably increasing the potential difference between the electrode and the nanotube to remove material from a portion of a nanotube thereby shaping the nanotube to exhibit a desired contour.
To provide precise removal control, material removal may be carried out via successive removal of layers from the exterior surface of the nanotube. Shaping of the nanotube contour, via successive layer removal or otherwise, can be monitored from the electrical characteristics of the nanotube. Preferably, the shaping of the nanotube is carried out xe2x80x9cblindly.xe2x80x9d That is, monitoring of the electrical characteristic is carried out alone, without the accompaniment of any other technique such as microscopy. While any of a number of shaping electrodes may be employed, the preferred shaping electrode comprises an additional nanotube. This additional nanotube may be comprised of the same material as the nanotube to be shaped or may be comprised of a different material. Typically, the additional nanotube is larger than the nanotube to be shaped.
Although the method can be employed to shape the nanotube so that the shaped nanotube exhibits any desired contour, a tapered contour can be advantageously employed in a number of contexts and is thus preferred.
In another embodiment, the invention relates to a method for shaping a solid article. The method involves the use of a shaping electrode comprising a nanotube to remove material from a portion of a solid article such that the solid article is controllably shaped to exhibit a desired contour. As before, material removal is carried out while the solid article and the shaping electrode are under a potential difference. Typically, the solid article is comprised of material having a layered structure. Thus, material removal involves removing one or more outer layers of material from the solid article.
In still another embodiment, the invention relates to a method for shaping a nanotube by removing material from a portion of the nanotube through controlled application of energy until the nanotube exhibits a desired contour, wherein material removal is carried out without need for a chemical etchant.
In a further embodiment, the invention relates to a nanotube having a controllably shaped contour and a varying cross-sectional dimension along the longitudinal axis. Typically, the nanotube is comprised of a material having a layered structure such as carbon, boron nitride, boron carbide, carbon nitride, carbon boron nitride, or a transition metal chalcogenide. Preferably the material is carbon.
When the nanotube comprises material having layered structure, it is typical for at least a portion of the nanotube to have 1 to about 1000 layers, with 1 to about 100 layers preferred, and about 2 to about 50 layers most preferred.
The nanotube may be made to order to conform to any of a number of desired contours. Often, it is desired that the contour be tapered. Thus, the cross-sectional dimension of the nanotube along the longitudinal axis may vary by up to about 100-fold but more typically varies by about 2-fold to about 10-fold. The nanotube may be substantially symmetric or asymmetric about the longitudinal axis. When it is important to ensure that the contour is smooth, it is preferred that the nanotube exhibits substantially no exfoliation.
Such a nanotube may represent component of a catalyst, an electrode such as a biological cell electrode, an electronic system, a mechanical system, an emission tip such as an electron field emission tip or a scanned probe microscope emission tip, or a probe for biological insertion. The probe, e.g., may be hollow tube to allow the delivery of a compound through the lumen of the tube.
In a still further embodiment, the invention relates to a three-dimensional object comprising a material having a layered structure and having a controllably shaped exterior contour, wherein at least one dimension of the solid article does not exceed about 100 nm in length.